This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer bridge, or the like, ignited by a high voltage pulse.
Such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass or primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as shredded zirconium or hafnium foil, and a combustion-supported gas, such as oxygen, at an initial fill pressure of several atmospheres.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, as for example, from a piezoelectric crystal) across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
The fabrication and testing of a number of different ignition structures has shown several problem areas that are peculiar to high voltage type flashlamps, and which are familiar to those skilled in the art of flashlamp design. For example, random location of the shreds of metallic combustible can cause short circuiting of the lead-in wires or interfere with the intended electrical breakdown path through the primer.
An example of a prior art lamp structure directed to overcoming some of those problems is described in U.S. Pat. No. 3,873,260 to Cote wherein one of the lead-in wires of the ignition mount is recessed in a glass insulating sleeve which is sealed to the press at one end and open at the other end. The other lead-in wire is formed so that it rests against and terminates slightly above the open end of the sleeve. The mass of primer material is disposed to cover the open end of the sleeve and bridge the ends of the lead-in wires. The glass sleeve has a side vent opening for the purpose of avoiding air entrapment during primer application to assure the primer material reaches the sleeved lead. Such a vent hole, however, introduces a degree of added cost and exposes the sleeved lead-in wires to a possible shred shorting condition. Consequently, an alternative approach that has been employed is to use a continuous sleeve, with no venthole. But this last-mentioned mount design also has some apparent shortcomings. The fact that the sleeved lead-in wire is recessed causes problems with primer bridging. It is necessary to use air pressure to force primer into the glass sleeve to contact the lead. This method consists of a seal connecting the top edge of the primered bottles and using the same seal as a means to force primer into the sleeve. Poor sealing of the bottle caused by a slight chip in the glass, worn or torn sealing edge, etc., can cause splashed primer and primer not contacting the lead in the sleeve. Another criticism of the prior construction is the possibility of shreds getting into the sleeve opening. Since the primer is being forced into the sleeve, an opening can appear in the primer, enhancing the possibilities of shred shorts. Further, the glass insulating sleeve is expensive and requires a special mount shaped for proper support and dimensional control. This can result in an unbalanced stress condition after sealing into the glass envelope, which then requires special annealing.
Another prior art lamp structure of interest is described in U.S. Pat. No. 3,884,615 of Sobieski wherein the two lead-in wires of the ignition mount are sealed into a doughnut-shaped glass bead which is open at both ends. The central opening in the lead is filled with a mass of primer material which bridges the lead-in wires. This construction uses the bead as a shield to keep the combustible fill away from the bare lead wires below the bead. The bead obviously must be smaller than the inside diameter of the lamp envelope. However, this creates a space for strands of fill to slip past the bead and come in contact with the lead wires, thereby shorting out the system and rendering the lamp inoperable. The close proximity of the bead and lamp envelope requires precise mount placement in order to prevent the bead from being sealed in the lamp envelope, thus weakening the final product. The Sobieski patent does disclose alternatives to counter the shred short problem, such as the use of a sleeve below the bead or special bead shaping, but such design adds to the cost of a bead structure, which is in itself comparatively expensive, and introduces additional manufacturing problems. Primer application to this structure is also difficult, requiring the use of a dip rod technique as compared to a dip cup that can be used with the construction of the aforementioned Cote patent. Another difficulty with this construction is the additional cooling time required in lamp pressurizing due to the slow transfer of heat from the bead through the inner lead wires.